Engine Controls - Tests W/o Codes Porsche 911 Carrera 993

INTRODUCTION

Before diagnosing symptoms or intermittent faults, perform steps in BASIC TESTING and TESTS W/CODES articles. Use this article to diagnose driveability problems existing when a hard fault code is not present or vehicle is not equipped with a self-diagnostic system.

Note. Some driveability problems may have been corrected by manufacturer with a revised computer calibration chip or computer control unit. Check with manufacturer for latest chip or computer application.

Symptom checks can direct the technician to malfunctioning component(s) for further diagnosis. A symptom should lead to a specific component or system test, or adjustment.

Use intermittent test procedures to locate driveability problems that DO NOT occur when the vehicle is being tested. These test procedures should also be used if an intermittent trouble code was present, but no problem was found during self-diagnostic testing.

Note. For specific testing procedures, see SYSTEM/COMPONENT TESTS article. For specifications, see ADJUSTMENTS or SPECIFICATIONS article.

SYMPTOM DIAGNOSIS

Symptom checks cannot be used properly unless the problem occurs while the vehicle is being tested. To reduce diagnostic time, ensure steps in BASIC TESTING and TESTS W/CODES articles were performed before diagnosing a symptom. Symptoms available for diagnosis include the following

1. Engine Will Not Start
2. Engine Hard To Start
3. Erratic Idle
4. Poor Pick-Up (Acceleration)
5. Misfire
6. High Fuel Consumption
7. Engine Hesitation
8. Poor Power Output
9. Poor Hot Starting
10. Diagnosis Not Possible

ENGINE WILL NOT START

1. Check fuel pressure.
2. Check ignition system.
3. Check intake system for leaks.
4. Check DME relay.
5. Check voltage to DME control unit.
6. Check ground circuits and connectors.
7. Check engine temperature sensor and circuit.
8. Check engine speed/reference mark sensor and circuit.
9. Check idle speed control activation circuit.
10. Check fuel injector(s) and circuit(s).
11. Check DME control unit.

ENGINE HARD TO START

1. Check fuel pressure.
2. Check ignition system.
3. Check intake system for leaks.
4. Check DME relay.
5. Check ground circuits and connectors.
6. Check idle switch and circuit.
7. Check idle speed control activation circuit.
8. Check engine coolant temperature sensor and circuit.
9. Check engine speed/reference mark sensor and circuit.
10. Check fuel injector(s) and circuit(s).
11. Check fuel tank venting solenoid.
12. Check carbon canister.
13. Check DME control unit.

ERRATIC IDLE

1. Check fuel pressure.
2. Check fuel pressure regulator.
3. Check ignition system.
4. Check intake system for leaks.
5. Check DME relay.
6. Check ground circuits and connectors.
7. Check oxygen sensor and circuit.
8. Check oxygen regulation stop circuit.
9. Check MAF sensor and circuit.
10. Check idle switch and circuit.
11. Check idle speed control activation circuit.
12. Check intake air temperature sensor and circuit.
13. Check engine coolant temperature sensor and circuit.
14. Check fuel tank venting solenoid.
15. Check fuel injector(s) and circuit(s).
16. Check DME control unit.

POOR PICK-UP (ACCELERATION)

1. Check fuel system for leaks.
2. Check fuel pressure.
3. Check ignition system.
4. Check exhaust system for restrictions.
5. Check air intake system for leaks.
6. Check DME relay.
7. Check ground circuits and connectors.
8. Check oxygen sensor and circuit.
9. Check oxygen regulation stop circuit.
10. Check MAF sensor and circuit.
11. Check intake air temperature sensor and circuit.
12. Check engine coolant temperature sensor and circuit.
13. Check engine speed/reference mark sensor and circuit.
14. Check Hall Effect sensor and circuit.
15. Check fuel tank venting solenoid.
16. Check fuel injector(s) and circuit(s).
17. Check DME control unit.

MISFIRE

1. Check fuel pressure.
2. Check ignition system.
3. Check intake system for leaks.
4. Check ground circuits and connectors.
5. Check charging system voltage.
6. Check voltage to DME control unit.
7. Check oxygen sensor and circuit.
8. Check oxygen regulation stop circuit.
9. Check engine speed/reference mark sensor and circuit.
10. Check fuel tank venting solenoid.
11. Check fuel injector(s) and circuit(s).
12. Check DME control unit.

HIGH FUEL CONSUMPTION

1. Check fuel system for leaks.
2. Check fuel pressure.
3. Check exhaust system for restrictions.
4. Check intake system for leaks.
5. Check oxygen sensor and circuit.
6. Check oxygen regulation stop circuit.
7. Check idle switch and circuit.
8. Check full throttle switch and circuit.
9. Check intake air temperature sensor and circuit.
10. Check engine (coolant) temperature sensor and circuit.
11. Check MAF sensor and circuit.
12. Check resonance plate.
13. Check Hall Effect sensor and circuit.
14. Check knock sensors and circuits.
15. Check fuel injector(s) and circuit(s).
16. Check DME control unit (knock control).

ENGINE HESITATION

1. Check fuel pressure.
2. Check fuel pressure regulator.
3. Check ignition system.
4. Check intake system for leaks.
5. Check DME relay.
6. Check oxygen sensor and circuit.
7. Check full throttle switch and circuit.
8. Check intake air temperature sensor and circuit.
9. Check engine coolant temperature sensor and circuit.
10. Check engine speed/reference mark sensor and circuit.
11. Check MAF sensor and circuit.
12. Check Hall Effect sensor and circuit.
13. Check knock sensors and circuits.
14. Check fuel injector(s) and circuit(s).
15. Check DME control unit (knock control).

POOR POWER OUTPUT

1. Check fuel pressure.
2. Check fuel pressure regulator.
3. Check ignition system.
4. Check intake system for leaks.
5. Check MAF sensor and circuit.
6. Check DME relay.
7. Check ground circuits and connectors.
8. Check charging system voltage.
9. Check voltage to DME control unit.
10. Check oxygen sensor and circuit.
11. Check oxygen regulation stop circuit.
12. Check idle switch and circuit.
13. Check intake air temperature sensor and circuit.
14. Check engine coolant temperature sensor and circuit.
15. Check engine speed/reference mark sensor and circuit.
16. Check fuel tank venting solenoid.
17. Check fuel injector(s) and circuit(s).
18. Check DME control unit.

POOR HOT STARTING

1. Check fuel pressure.
2. Check ignition system.
3. Check carbon canister.
4. Check fuel tank venting solenoid.
5. Check intake system for leaks.
6. Check DME relay.
7. Check oxygen sensor and circuit.
8. Check idle speed control activation circuit.
9. Check intake air temperature sensor and circuit.
10. Check engine coolant temperature sensor and circuit.
11. Check engine speed/reference mark sensor and circuit.
12. Check fuel injector(s) and circuit(s).
13. Check DME control unit.

DIAGNOSIS NOT POSSIBLE

1. Check voltage to DME control unit.
2. Check ground circuits and connectors.
3. Check diagnostic connector inputs to DME control unit.
4. Check DME control unit.

INTERMITTENT PROBLEM DIAGNOSIS

Intermittent fault testing requires duplicating circuit or component failure to identify the problem. These procedures may lead to computer setting a Diagnostic Trouble Code (DTC) which may help in diagnosis.

If problem vehicle does not produce a DTC, monitor voltage or resistance values using a DVOM while attempting to reproduce conditions causing intermittent fault. A status change on DVOM indicates a fault has been located.

When monitoring voltage, ensure ignition switch is in ON position or engine is running. Ensure ignition switch is in OFF position or negative battery cable is disconnected when monitoring circuit resistance. Status changes on DVOM during test procedures indicate area of fault.

TEST PROCEDURES

Intermittent Simulation

To reproduce conditions creating intermittent fault, use following methods

1. Lightly vibrate component.
2. Heat component.
3. Wiggle or bend wiring harness.
4. Spray component with water.
5. Remove/apply vacuum source.

Monitor circuit/component voltage or resistance while simulating intermittent. If engine is running, monitor for self-diagnostic codes. Use test results to identify a faulty component or circuit.

Poses Online

See also:
• BASIC TESTING
• TESTS W/CODES
• SYSTEM/COMPONENT TESTS
• ADJUSTMENTS
• SPECIFICATIONS